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Abstract

Electrochemical devices based on conducting polymers suffer from short operating lifetimes and incomplete utilization of the polymer's capacitance. These problems are common to the investigation of conducting polymers for energy storage applica- tions A primary cause of the short lifetimes could be due to poor interaction between the highly polar electrolyte system and the lower polarity of the con- ducting polymer. If the degree of interaction between polymer and electrolyte is low, then phase separation may occur when the conducting polymer is cycled between the doped and undoped states during charging and discharging. The result will be a dwindling capacity of the conducting polymer due to charge and/or solvent trapping. This paper describes a unique approach to conducting polymer stabilization through the use of electrolyte additives.

Copyright

Motorola Inc. December 1997

Country

United States

Language

English (United States)

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m MOTOROLA Technical Developments

ADDITIVES FOR ENHANCED PERFORMANCE OF CONDUCTING POLYMERS IN ELECTROCHEMICAL CELLS

Electrochemical devices based on conducting polymers suffer from short operating lifetimes and incomplete utilization of the polymer's capacitance. These problems are common to the investigation of conducting polymers for energy storage applica- tions A primary cause of the short lifetimes could be due to poor interaction between the highly polar electrolyte system and the lower polarity of the con- ducting polymer. If the degree of interaction between polymer and electrolyte is low, then phase separation may occur when the conducting polymer is cycled between the doped and undoped states during charging and discharging. The result will be a dwindling capacity of the conducting polymer due to charge and/or solvent trapping. This paper describes a unique approach to conducting polymer stabilization through the use of electrolyte additives.

A surfactant was used which acts as a "compati- bilizer" between the electrolyte system and the conducting polymer. The compatibilizer contains both polar and nonpolar chemical groups, the latter to interact with the conducting polymer and the for- mer to interact with the electrolyte. In this manner, there should be better ion and solvent transfer across the interface, resulting in increased operating lifetimes due to decreased amounts of charge and/or solvent trapping.

One method to measure the influence of additives is to monitor the size of the prepeaks in cyclic voltammograms of the conducting polymerielec- trolyte system with and without the additive. Prepeaks are indicators of charge and/or solvent that was trapped in the electrode and not released after reversing the current. Figure 1 shows the prepeaks for PEPT in 1.0 M TEABFJPC electrolyte. When the neutral, insulating polymer is oxidized (or reduced), the material becomes conductive. Ions from the electrolyte move into the polymer film to balance the charge. When the polymer is driven

back to its neutral state, the material again becomes insulating before all of the charge can be recovered. When the neutral polymer is driven to its other potential extreme, this trapped charge is recovered as a "prepeak" on the reduction (or oxidation) reaction. The size of the prepeaks approximates charge-discharge efftciency losses.